Flow Logic Laboratory

Ask ten builders what main jet to run on a red plate clone and you'll get ten answers, most of them some version of "go bigger than unrestricted." They're right. What almost nobody explains is why. The common answer is "the plate kills the signal, so you need more jet." That's half true, and the half that's missing turns out to be the whole story.

The Test:

One carburetor, four plates, and one set of gaskets. Everything was tested at 28" H2O depression. For each plate (Red .375, Green .425, Purple .500, Blue .550) and unrestricted, we measured two things:

1. Idle circuit signal: butterfly closed, same idle screw setting between plates, reading the pressure the pilot circuit sees.

2. Main circuit signal: wide open throttle, reading the signal pulled through the e-tube on the main jet, along with total airflow in CFM.

The Idle Circuit Plate Doesn't Matter

Plate "H2O

Blue 15.7

Purple 15.8

Green 15.7

Red 15.4

From blue to red, the idle circuit signal changes by about 2.5%. Fuel flow through a jet goes with the square root of signal, so that's roughly a 1% difference in fuel. Stepping up to .027 from .026 pilot jet size changes fuel by about 3.8%. The plate's effect on the low side is smaller than a single jet step.

This makes sense once you picture it. With the butterfly closed, the butterfly is the restriction. The gap around it is tiny compared to even a red plate's .375" hole, so the plate isn't doing anything yet. The plate only starts to restrict once the butterfly opens past roughly 40° on a red plate, and about 60° on a blue.

Bottom line: you don't need a different low speed jet for each plate class according to the flow bench readings.

The High Side: Where It Gets Interesting

Fuel signals were tested through the e-tube and represent "H20 at the e-tube, % of e-tube signal to test pressure, and CFM at 28".

Plate E-tube Test % CFM

Red 1.7 6.0% 10.76

Green 2.7 9.8% 13.59

Purple 5.1 18.1% 19.1

Blue 7.3 26.2% 23.1

None 22.8 81.3% 42.5

A red plate drops the main jet signal from 22.8" to 1.7", more than 90% gone. So you'd expect the red plate to need a huge jet, right?

Here's the surprise: if you do the math on airflow and signal alone, the plates need a smaller jet, not a bigger one.

The plate cuts the airflow too. A venturi carb is naturally self-metering: signal drops roughly with the square of airflow, and fuel flow drops with the square root of signal. So when air goes down, fuel goes down right along with it. Run the numbers on our data using signal and CFM alone, starting from a .038 unrestricted main, and every plate calls for about a .037.

That doesn't match reality. Everyone who has built a red plate engine knows it wants more jet. So what's missing?

The Missing Piece: Lifting the Fuel

Before a single drop of fuel comes out of the e-tube, the carb has to lift the fuel from the level in the bowl up to the spill point. That lift has a fixed cost: it takes a certain amount of signal just to hold the fuel column up, and only the signal left over actually pushes fuel through the jet.

On our carbs, the fuel lift from the float level to the spill point measures about .750" to .850". Gasoline is about 74% the density of water, so that lift costs roughly 0.56 to 0.63 inches of water of signal.

Now look at what that does to each plate:

Unrestricted: 22.8" of signal. Losing 0.6" to the lift is about 3%. It barely matters.

Blue: 7.3" of signal, about an 8% loss.

Red: 1.7" of signal. The lift eats over a third of it.

That's the real reason plate engines need bigger jets. The lower signal alone doesn't do it, because the air drops right along with the signal. It's the fixed cost of lifting the fuel, which becomes a big slice of a small signal.

It also explains why the pilot jet stays the same according to the math. The idle circuit sees about 15.5" of signal, so the lift is only a few percent of it on every plate.

The Bench Numbers

Take the lift required to pull fuel through the e-tube out of the signal and run the math again to hold the same air/fuel mixture as a .038 unrestricted engine:

Plate Equal-Mixture Main Jet

Red .040–.041

Green .039

Purple .038

Blue .038

None .038 (baseline)

What stands out:

Blue and purple still have enough signal that the lift hardly matters. They run right with an unrestricted engine.

Green wants about one jet size up.

Red wants about three sizes up. It's also the most sensitive to the exact lift height, which is why red plate jetting is so touchy.

These line up closely with what experienced builders run. Many run green and purple a size richer than equal mixture, which is a reasonable power or safety margin, but the bench shows the base requirement behind it.

Why This Matters to You

Don't swap pilot jets between plate classes chasing something that isn't there. Get your idle right and only adjust if the particular engine build requires it.

Red plate engines are the most sensitive to fuel. A heavier fuel costs more signal to lift, and on a red plate that's a noticeable share of the tiny fuel signal available.

These are starting points, not gospel. A flow bench measures steady airflow, and a running engine pulses. Air density, timing, and your particular engine will move the final number. But now you know why the numbers go the way they do, and you'll get to the right jet faster.

Always check your current rulebook for what's legal in your class and consult your engine builder for specifics on jetting.

All testing was done on the Flow Logic FL-220 flow bench, using its aux channel to read carb signals alongside CFM.

Stop Guessing, Start Flowing.

October 2026